The present invention relates to process control apparatus. More particularly, it relates to improved digital input circuitry for use with a computer based process control system in which the input is representative of a count of process related events.
In the art of industrial process control, there have been provided systems wherein various parameters of the process are measured, compared with the desired value, and a difference used to derive a control or input signal. In a number of cases, both the input, or measured variable signals and the output signal are in the form of analog signals. In some cases, however, the primary sensing elements produces signals representative of measured variables in the form of digital signals. In such systems, a great number of input circuits share a single process control computer. Again, in certain situations, the digital signals represent a count of recurrent events in the course of the controlled process. The parametric signals may be in the nature of a form A switch closure or a form C switch closure and may represent such parameters as the movement of a conveyor belt or object thereon, a turbine speed, a fluid flow or the like. To be meaningful to the computer, the events must be counted and the results presented in digital form.